1. Field of the Invention
This invention relates generally to apparatus for sensing and measuring changes in inclination, and more particularly to electronic apparatus for providing a digital readout of a capacitive inclination sensor.
2. Description of the Prior Art
Apparatus employed in the prior art for sensing and measuring changes in inclination and providing a digital readout therefor have generally relied on a bubble level type sensor of the type having an arcuate glass chamber enclosing an electrolyte with electrodes immersed therein so that an inclination of the sensor results in a change in resistance between the electrodes. By sensing the resultant change in magnitude and direction of inclination with appropriate phase and amplitude sensitive circuitry, a very high degree of sensitivity may be provided so that very small changes in inclination can be measured with high precision.
One apparatus of this type is described in U.S. Pat. No. 4,167,818, "Electronic Inclination Gauge", issued to Robert Cantarella, et al, on Sept. 18, 1979. A further such device is disclosed in U.S. Pat. No. 4,377,912, "Apparatus for Sensing and/or Measuring Changes in Inclination", issued on Mar. 29, 1983 to Armik A. Hakhverdian. However, these prior art devices depend on electrolytic levels which employ precious metal electrodes and are inherently expensive to fabricate and calibrate. Further, the circuitry required to attain a digital readout is complex and expensive, and heretofore has not allowed a user to choose between alternate readout units, depending on his application, nor to preselect a zero reference which differs from the earth's gravity vector for convenience in measuring a change in inclination of a tilted surface.
The Hakhverdian apparatus provides a sensor which presents a capacitance which is a function of its inclination. The capacitor is used as an element of an oscillator whereby a change in capacitance varies the oscillator frequency which in turn may be applied to control an indicating instrument. However, an apparatus of this type suffers from instability and temperature drift due to its analog nature and therefore is unsuitable for precision measurements of high stability.
The present apparatus is intended for use with a digital interface, of the type described in Differential Capacitance Detector Ser. No. 696,626, filed Jan. 31, 1985, by H. L. Swartz, et al, and assigned to the Assignee of the present invention. The detector described therein provides a pulsed output, wherein the pulse width is a function of the inclination of a variable capacitance sensor, and which is adaptable to an all-digital readout apparatus. Thus, the instability and sensitivity to environmental changes of the prior art are readily overcome.